1. Field of the Invention
The present invention relates to an apparatus for measuring a distance using light radiation and a surveying instrument having an automatic focusing device.
2. Description of the Related Art
In a surveying instrument having a distance measuring function, such as an optical distance meter or a total station, etc., a sighting telescope is oriented toward a target point and a focusing knob is manually rotated to focus the telescope on the target point.
However, the manual focus adjustment prevents an operator from concentrating on the sighting operation, thus increasing the length of time necessary to complete the focusing operation.
In a surveying instrument having a distance measuring function, such as an optical distance meter or total station, etc., object measuring light emitted from a light source, such as a light emitting diode (LED) or laser diode, toward an object (aiming object) to be measured is reflected from the object before being received, to detect the distance between the surveying instrument and the object. The distance is detected using a phase difference detection method in which a phase difference or time difference between the emitted light and the received light is detected. A known optical distance meter is provided with a sighting telescope which makes the measuring light incident upon the object to be measured.
However, in conventional optical distance meters (an optical range finder or an electronic distance meter), the emission angle of the measuring light (detection beam) is restricted when detecting the distance of an object located from a close distance to a long distance, for example, 1 KM or more. To this end, if the object deviates from the center of the field of view of the sighting telescope when the object is viewed through the sighting telescope, there is a possibility that little or no measuring light reaches the object to be measured, resulting in non-measurement or an incorrect measurement.